Fluorescent additives provide an excellent leak detection technique for determining the site of leakage of an operating fluid from a working system. Operating fluids such as lubricants, hydraulic fluids, heat transfer fluids, and refrigerants are treated with a dye additive which fluoresces when illuminated by suitable ultraviolet or visible blue light.
Fluorescence is generally understood to be a property that enables certain materials to absorb light energy and radiate visible light at a longer wavelength than the absorbed light. According to generally accepted theory, electrons in fluorescent materials are excited upon being illuminated by light energy of a specific wavelength, and light energy of a longer wavelength is radiated from these materials as the electrons return to the unexcited or ground state. The specific excitation and radiation wavelengths are characteristics of the particular fluorescent materials. The apparent brightness of a fluorescent material's luminescence is dependent on the wavelength emitted by the material and the intensity of the incident radiation that excites the material. For example, a fluorescent dye which has its excitation peak at a specific wavelength may quickly emit a much reduced luminescence as the wavelength of incident light deviates from the excitation peak, and will lose the ability to fluoresce when the incident light does not have enough energy within the specific excitation range.
The visibility of the fluorescent response is much increased when the intensity of other visible light is reduced, so that the fluorescent response is not masked or washed-out by other light. Thus, ultraviolet/blue leak detection lamps directed in otherwise dark conditions at an operating system containing a UV/blue responsive fluorescent dye will reveal leak sites which glow against the dark background.
The most common UV/blue fluorescent leak detection dyes used today are either perylene-based fluorescent compounds or naphthalimide-based fluorescent compounds. Perylene dyes produce an intense yellow fluorescent response when exposed to incident radiation in a band of the electromagnetic spectrum which includes the long wave ultraviolet (UV-A) wavelength range of about 315 nm to about 400 nm, with a strong peak between about 340 to 375 nm. Long-wave ultraviolet is also referred to as "black light", as it includes a small segment of the visual violet range. Naphthalimide dyes fluoresce a brilliant green when exposed to incident radiation of visible violet/blue light. The visible violet/blue range extends from about 400 nm to about 480 nm within the electromagnetic spectrum. Both perylene and naphthalimide dyes are useful for leak detection in oil-based working fluids or fluids in which oil is miscible.
The various inspection lamps used to illuminate the exterior of the fluid-containing portions of the operating systems can be grouped by emission wavelength into three general types: (a) UV filter (see FIG. 1), (b) UV/BLUE glass filter (FIG. 2) and (3) broad spectrum UV to blue thin film filters (FIG. 3).
The UV filter lamps emit long wave ultraviolet light which provides optimal energy for use with the perylene dyes. These high intensity/ narrow band UV lamps will also emit enough energy in the naphthalimide dyes' broader excitation band to produce a fluorescent response. However, these high intensity lamps tend to be larger than those used to provide visible light and can be cumbersome in situations where working space is very limited.
The lower intensity lamps with UV/BLUE glass filters provide visible violet/blue illumination are optimally suited for use with the naphthalimide dyes, whose excitation peak lies within this wavelength band. These lamps may cause the perylene dyes to fluoresce, though only poorly. These lamps are also typically less expensive and more compact than high intensity lamps.
The third type is the relatively new broad spectrum lamps which use thin film filters to pass a substantial amount of visible violet to blue light as well as long wave ultraviolet light. These lamps are the most versatile, as they can be used effectively with both perylene and naphthalimide fluorescent dyes, and can provide intense illumination even in compact form. Being relatively new, however, there are fewer of these lamps in use than the other types.
The sensitivity of the human eye to low Intensity light is greatest for wavelengths between about 540-570 nm (yellow green). Perylene dyes emit an intense, but narrow band, fluorescent response, which has a main intensity peak between about 520 and 550, and a smaller peak between about 560 to 580. Thus, the color of the perylene fluorescent response is close to that wavelength range which the human eye is most sensitive, and the perylene dyes exhibit a more intense fluorescence than that of the naphthalimide dyes. Under ideal conditions in clean, clear fluids, perylene dyes provide a superior fluorescence to that of naphthalimides.
However, what the leak inspector sees is the combination of the fluorescence of the dye and any natural fluorescence of the fluid. This combination may "wash out" the apparent brightness. Many of the lubricants to which fluorescent leak detection dyes are applied have some natural fluorescent response to UV/visible blue light energy. These fluids may also have a color or may be tainted with contaminants which can mask the fluorescent response of the leak detection dye. The combination of a natural fluorescence from host fluids, and the presence of contaminants (such as dirt or combustion residue) may mask the fluorescent response of a perylene dye.
Naphthalimide dyes emit a broad fluorescent response with an intensity peak between about 480-520 nm. The naphthalimide dyes do not fluoresce as brilliantly as the perylenes, but the green fluorescence of a naphthalimide dye will not be as noticeably diminished by the natural fluorescence of a host fluid.
The majority of fluorescent leak detection dyes presently being marketed for use in oil-based working fluids, or in fluids in which oil is soluble, employ perylene-based dye materials. These dyes are applied in lubricating systems, fuel systems, transmission fluids, and hydraulic fluids. Naphthalimide dyes are primarily used in applications within some air conditioning and refrigeration systems where polyolester or polyalkylene glycol-based lubricants are used with R-134a or similar refrigerant.
Typical automotive fluids encompass a wide range of physical properties and include air conditioning refrigerant, engine lubricating oil, transmission fluid, brake fluid, power steering fluid, radiator coolant, diesel oil, and gasoline. In an auto repair facility, where diagnostic leak detection using fluorescent dyes is commonly performed on both air conditioning and other fluid systems, there is generally only one inspection lamp available for leak detection purposes. This light source may not be optimally, or even altogether, effective on the dye in the target fluid.
It would be desirable to have a fluorescent dye for the various types of working fluids which is effective when used with any of the available inspection lamps. It would also be desirable to have a dye which is effective in all automotive working fluids.
Thus, one objective of this invention is to create blended dye compositions of perylene and naphthalimide dye in various proportions to create effective dyes for particular fluids with a substantial fluorescent response to illumination by the entire range of inspection lamps. Another objective is to create a single all-purpose blend that is satisfactory for all automotive fluids and all inspection lamps.